In force-measuring devices, for example in a type of weighing cells that operate according to the principle of electromagnetic force compensation, also known as electromagnetic force restoration or EMFR, the weight force of the weighing object is transmitted either directly or by way of one or more fulcrum-supported force-transmission levers to an electromechanical measurement transducer. The measurement transducer generates a compensation force that matches the weight force of the weighing object and delivers an electrical signal which is processed and displayed by an electronic aggregate, the signal-processing unit.
An EMFR weighing cell includes in most cases a parallelogram mechanism with a stationary parallel leg and a movable parallel leg which is connected to the stationary parallel leg by two parallel guides and serves as load receiver. In EMFR weighing cells with lever systems the weight force is transmitted, by way of a coupling element that rigidly maintains its length but is flexible in bending, to a balance beam that is pivotally supported on the stationary parallel leg. The purpose of such an arrangement is to make the weight force exerted by the applied load sufficiently smaller through lever reduction, so that the measurement transducer generating the compensation force will be able to produce a measurement signal representing the weight force. According to the state of the art, the connections between the individual elements in high-resolution weighing cells are designed as flexure pivots. A flexure pivot defines an axis of rotation between the two coupled elements. In a weighing cell constructed as a materially continuous unit, also called a monolithic or monobloc weighing cell, the flexure pivots can be formed as narrow material connecting portions.
In a type of EMFR weighing cells where the weight force is compensated directly, i.e. without a lever reduction of the compensation force, the parallel-guiding mechanisms are in most cases configured as spring elements, specifically as flexible links or diaphragm springs. In weighing cells of this type, which are also referred to as direct measuring systems, the measurement transducer counteracts the weight force of the load with a compensation force of equal magnitude.
The measurement transducer of an EMFR weighing cell of the current state of the art is normally configured as a current-conducting coil immersed in the air gap of a permanent magnet, so that the weight force of the weighing object can be determined from the corresponding electrical variable, i.e. the coil current I. As the weight force is proportional to the compensation force and the latter is, in turn, proportional to the coil current I, it follows that the coil current I is also proportional to the weight force and thus proportional to the mass of the weighing load. When the weighing cell is in equilibrium, this relationship can be expressed by the equation (not accounting for temperature effects):F′=f(I)=k×I where                F′: is the calculated weight force of the mass m being weighed;        K is the transfer constant, and        I is the coil current.        
The transfer constant k is specified in the design for every type of weighing cell and is stored in a processing unit for the calculation of the compensation force. In other words, the transfer constant depends on the load-receiving system of the weighing cell and describes the conversion of a measured coil current into a force F′.
In measurement transducers of the type that is normally used in an EMFR weighing cell, the current-conducting coil moves in the air gap of the permanent magnet under the influence of the Lorentz force, i.e. the force acting on an electrical charge that moves in a magnetic field which in this case manifests itself as the compensation force of the coil. A current which flows in an electrical conductor such as a coil winding in a transverse direction to the trajectories of a surrounding magnetic field will cause a force to act on the conductor, and thus on the coil of the measurement transducer.
The magnetic field of the permanent magnet should ideally be homogeneous, a condition which is not met in practice in most cases. The magnetic field can vary to some extent for different positions of the coil relative to the permanent magnet. In other words, with a current of a given magnitude the compensation force generated by the coil is position-dependent. This needs to be taken into account in the calculation of the weighing result. If the balance beam is not in the equilibrium position for which the transfer constant k was determined, but slightly above or below, the stored transfer constant will deviate from the actual transfer constant. If a weighing cell is exposed to shocks, vibrations and the like, the deviation of the transfer constant will affect the fast and precise determination of the weighing result and there can be deviations of the zero point. Particularly in dynamic checkweighing scales, this problem occurs with increased severity.
To mitigate this problem, electronic filters that act on the signal representing the compensation force are used in state-of-the-art force-measuring devices. In the operating state of a force-measuring device, the calculation of the compensation force is adapted to the ambient conditions by applying a correction based on temperature dependencies of the magnet and of the spring constants of the flexure pivots. Also taken into account are dynamic effects associated with switching the device on and with a change of the weighing load. Furthermore, time-dependent phenomena are also included in the compensation. The calculation of the weight force of the mass on the load receiver can be mathematically expressed as a function):F′=f(I,T,t).This function, which is also called transfer function, serves to convert the signals of the coil current and of different temperature sensors into a time-dependent output value which is presented on a display. Accordingly, the transfer constant is part of the transfer function. The coil current is regulated by a position-controlling unit according to the following function:I=f(F,z,T,t),which in addition to temperature effects (T) and dynamic effects (t) also takes possible interference parameters (z) into account.
A dynamic checkweighing scale is a system that serves to weigh products in a production line while they are moving over the scale, to classify the products into given weight classes, to sort and/or eliminate the products according to weight class. Checkweighing scales are used in a wide variety of applications. These include, for illustrative purposes only: checking for overweight and underweight products; complying with legal requirements for the net contents of packaged goods; reducing product waste by using the weight data collected by the checkweighing scale to adjust the settings of the filling machines; classifying products by weight; measuring and recording the performance of the production facility or the production line; and verifying the piece count based on weight.
The purpose of checkweighing scales is to weigh 100% of the products of a product line. In consequence, data for the entire production output are collected to count the number of units produced, to keep production lots traceable, or for production statistics.
A checkweighing system normally consists of an infeed conveyor, a weighing conveyor, an outgoing conveyor with a sorting device, and a weighing terminal with a user interface. The weighing conveyor, which is located between the infeed conveyor and the outgoing conveyor, is supported by a weighing cell which weighs the product as it moves over the weighing conveyor.
In the operation of a dynamic checkweighing scale, the product moves from the infeed conveyor belt to the weighing conveyor belt, where it is weighed while in motion, and then travels on by way of the outgoing conveyor. The time available to determine the weighing result depends on the length and the transport velocity of the weighing conveyor. The weighing conveyor belt, which is supported by the weighing cell, receives the weight of the product as it arrives from the infeed conveyor and is relieved again of the weight as the product changes over to the outgoing conveyor. In the continuous weighing of products, the weighing cell is therefore subjected to alternate loading and unloading. This alternating load leads to oscillations of the conveyor belt which have a noticeable effect on the weighing result because of the very short time interval during which the product rests completely on the conveyor belt.
To obtain a weighing result more quickly in the presence of oscillations or vibrations, a concept of using two weighing cells simultaneously is proposed in EP 0 430 695 A2. A first weighing cell receives the weighing load and, based on the compensation force generated, sends a corresponding signal to a processing unit. A second weighing cell has the function to send to the processing unit a signal that reflects the behavior of the first weighing cell under the same oscillations or vibrations with a standard weight. The processing unit subtracts the signal of the second weighing cell from the signal of the first weighing cell and thus cancels the oscillations and vibrations in the signal of the first weighing cell, whereby the weighing result is produced. This concept has the disadvantage that two weighing cells are needed for each weigh station, which adds significantly to the manufacturing cost. One also needs to keep in mind that the two load cells may not have exactly equal amounts of inertia, that the response to oscillations and vibrations may therefore be different for the two weighing cells, so that the weighing result cannot be corrected entirely.
Deviations of the zero point can also be found in microbalances, most of which have force-measuring cells operating according to the principle of electromagnetic force-compensation. These balances are capable of measuring a weighing load of 10 grams with a measurement resolution of 0.001 milligrams, i.e. with a precision of one part in 10 million. Therefore, even the smallest vibration originating from the environment, for example, elevators in the building, will be enough to cause a deviation of the zero point. The zero point deviation appears to the user as an increase of the indicated display value which returns to normal only gradually after the disturbance has subsided. In measurements that extend over a long time period, an event of this kind can render an entire measurement serious unusable.
Another approach to reduce the influence that oscillations or vibrations have on the weighing result involves setting the balance on a heavy table or support structure with an elastic damping system. This method is preferred at locations or weighing stations where microbalances are used. This solution, too, raises the procurement cost for setting up a weighing station.
Solutions according to the existing state of the art in which only the coil current is used as a basis to calculate the weighing result, as described for example in EP 0 359 978 A3 have the disadvantage that at the time when the weighing result is calculated from the coil current, the balance needs to have reached its settling position or, more specifically, the position where the linearized weighing result agrees with the measured coil current. Every time there is a disturbance such as for example an oscillation or vibration, the force-measuring device is destabilized from its exact settling position, and this can give rise to zero point deviations.
The term “oscillations” as used here refers to dynamic deviations of status variables of the system from a mean value, also called fluctuations. The term “vibrations” in the present context refers to periodically alternating movements of the system that are mostly in the intermediate to high frequency range and of low amplitude.
In US 2003/0229600 A1, a method is disclosed for the rapid weighing of objects, wherein a platform with a weighing cell delivers an output signal to an analog/digital converter. The resultant digital output signal is processed by way of a low-pass filter and analyzed by a microprocessor in order to determine the weight of objects on the platform. The smoothing of the measured coil current with suitable electronic filters is known as a means to stabilize the weighing result that is to be calculated. The drawback here is that the speed and the accuracy of the method depend on the filter parameters and are in most cases mutually exclusive, meaning that an accurate determination of the weighing result takes more time and, conversely, that a fast determination of the weighing result is less accurate.
The present invention has the objective to provide a method whereby the weighing result can be determined quickly and at the same time precisely.
In addition, the invention aims to provide the capability to quickly obtain a precise weighing result in the presence of strong vibrations and/or vibrations originating from the ambient environment.